i) Field of the Invention
The present invention relates to new man-made materials that reflect circular polarized visible light; more especially the invention relates to a solidified liquid crystal cellulose film; an article comprising the film supported on or embedded in a planar substrate; a process for producing the film and a novel dispersion.
ii) Description of Prior Art
Chiral nematic (cholesteric) liquid-crystalline phases are characterized by a unique helicoidal molecular orientation.
Chiral nematic phases are known to have particular and unique optical properties when the pitch is of the same order of magnitude as the wavelength of visible light.
Three major properties can be distinguished, as follows:
a) Reflection of light by constructive interference; PA1 b) Circular polarization of the reflected light and reflection of circular polarized light of the same handedness as the chiral nematic structure without change of handedness; PA1 c) Exceptional optical activity (rotation of transmitted plane-polarized light).
The first of these is not limited to materials with chiral nematic order. It is due to a periodic layered structure, and is also found in smectic liquid crystals, in optical security devices made of thin alternating layers of metals and ceramics as well as in multilayer film coextrusions, (Dobrowolski, J. A.; Ho, F. C. and Waldorf, A., Applied Optics, Vol. 28, No. 14, (1989)). When white light is directed onto such materials, only characteristic wavelengths are reflected and these vary with the viewing angle. These reflections are due to constructive interference which follows Bragg's law. The light that is transmitted will lack the reflected wavelengths and hence will show the complementary spectrum. Only if the periodic repeat distance or chiral nematic pitch is of the order of visible light divided by the mean refracture index of the material, will iridescent colours that change with the viewing angle be seen; beyond these limits the phenomenon must be detected with infrared or ultraviolet sensitive equipment, but it is still present.
This optical variation is impossible to reproduce by any printing or photocopying process. For this reason, multilayered ceramic materials based on this principle have been manufactured and used as security devices to prevent counterfeiting of bank notes as described by Dobrowolski referred to hereinbefore. However, these ceramic materials are costly to manufacture, do not adhere easily to paper products and when added to paper prevent recycling of the broke since they do not redisperse. They are nevertheless thought to be more attractive than liquid crystal based devices for this use, for reasons mentioned below.
The second and third optical properties of chiral nematic liquid crystals concern circular polarization and optical activity, De Vries, H., Acta Crystallogr., 4, 219 (1951), and Fergason, J. L., Molecular Crystals, 1, 293 (1966). Useful applications of these properties have been proposed in liquid crystal optics for laser systems, (Jacobs, S. D., "Liquid Crystals for Laser Applications", in Optical Materials Properties, CRC Handbook of Laser Science and Technology, Vol. IV, ed. Weber, M. J., pp 409-465 (1986)), and as optical storage devices, (Hikmet, R. A. M. and Zwerver, B. H., Liquid Crystals, 13, No. 4, 561 (1993)).
Chiral nematic liquid crystals with these properties have also been considered as optical security devices in the past, but they suffer from the obvious problem that, by definition, liquid crystals are fluid, so that some way has to be found to incorporate them in solids while maintaining the characteristic order of the fluid state. The materials have been encapsulated, as described in British Patent 1,387,389, dispersed in solids, or sandwiched between glass or polymer films, but their colours change with the temperature; this change of colour with temperature makes the materials useful as thermal sensors. Solids or gels in which the liquid-crystalline order is preserved may be prepared by photopolymerization, crosslinking or cooling polymeric chiral nematics below their glass transition temperatures and will show relatively stable colours as long as they are not heated above it. Some chiral nematic liquid-crystalline polypeptide solutions also preserve their ordering when dried, but their pitches are not in the visible spectrum, (Friedman, Emil; Anderson, Courtney; Roe, Ryong-Joon and Tobolsky, Arthur V., U.S. Nat. Tech. Inform. Serv., AD Rep., No. 74986, 10 pp. Avail. NTIS, from: Gov. Rep. Announce. (U.S.), 1972, 12(17), 54). Proposed applications for solidified chiral nematic liquid crystals include security devices as in EP 435,029, decorative coatings as in BE 897,870; BE 897,871; U.S. Pat. No. 4,614,619; U.S. Pat. No. 4,637,896; BE 903,585 and DE 3,535,547, partially reflective films for car windows as in JP 01 61,238 and JP 01,207,328, or optical filters as in JP 61,170,704, in L.C.D.'s and information storage devices as in JP 01,222,220; JP 02,16,559 and EP 357,850.
Thus, many compositions and applications of chiral nematic liquid crystals are known. In all the above examples, the materials are composed of helicoidally oriented molecules or segments of molecules. Recently, aqueous suspensions of cellulose and chitin crystallites (fragments of microfibrils obtained by acid hydrolysis) were found to form chiral nematic liquid-crystalline phases by rapid and spontaneous self-assembly, (Revol, J.-F.; Bradford, H.; Giasson, J.; Marchessault, R. H. and Gray, D. G., Int. J. Biol. Macromol., 14, 170 (1992)), and to align with the chiral nematic axis parallel to strong magnetic fields, (Revol. J.-F.; Godbout, L.; Dong, X. M.; Gray, D. C.; Chanzy, H. and Maret, G., Liquid Crystals, in press). These phases are quite distinct from the molecular chiral nematics described above, in that the elements in the helicoidal arrangement were of colloidal dimensions, at least an order of magnitude larger than the molecules in most previously known chiral nematic suspensions. They are also distinct from previously known suspensions of cellulose crystallites (U.S. Pat. No. 2,978,446 and Marchessault, R. H.; Morehead, F. F. and Walter, N. M., Nature, 184, 632 (1959)), for which there is no evidence of their forming chiral nematic ordered phases.
Preliminary attempts to prepare solids from chiral nematic suspensions gave materials with chiral nematic pitch much greater than that necessary to reflect visible light (Revol et al, supra). Not surprisingly, no reflection colours were observed.
In fact, no prior procedure is known by which the final pitch obtained by drying such preparations can be controlled.